i’m an architect doing an octagon sunroom addition in nj. the room is a true octagon about 21′-0″ across with an 11/12 roof pitch. i don’t want to have collar ties.
do you have any recommendations on the roof structure?
should i use a steel connector to pick up the 8 hip rafters at the peak? simpson doesn’t make one- do you know where i can obtain that?
i was also thinking of doing a steel tie rod around the perimeter where the hip rafters meet the wall plate to create a ring and keep the walls from spreading. is this a good idea?
Replies
Consider using the top plate of the exterior walls as a built-up beam.
Typically, each segment of a framed 2x6 exterior wall will have a 2x6 top plate. Then, to lock the walls together, another 2x6 cap plate is nailed over that.
For your situation, think of adding additional lumber, as required, to turn the cap plate into, essentially, a beam laid on its side...which will resist lateral deflection instead of vertical deflection.
If you don't have the ability, an engineer can design it for you. If, for some reason, built-up 2x6's won't work and you need something along the lines of a 2x8, the walls can still be framed as 2x4 or 2x6. The regular top and cap plates can be 2x4 or 2x6. The built up beam can be 2x8, or as required, with the additional depth of the lumber hidden behind a soffit box on the interior.
Should elevations dictate, the top and cap plates, as well as additional "laminations", if needed, can all be 2x8 or as required. Again, the added depth can be hidden behind the trim.
I've called this a 'ring beam' in the past. I don't know if there is an official name for it.
We have a similar detail - we call it a tension ring. The 2-2x6 top plate is essentially turned into a beam on its side like you said. Our engineer worked out a detail with 2-3/4" plywood gusset plates at the corners to tie all the facets together. Each hip beam (usually a 4x) is lagged into the tension ring.
you know i like steel... a 4x6 x 5/16 angle with the 6" on the flat welded around the top plate and capped/lag bolted through with another 2x6 or even a 2x12 cap..would/should work... there was an artical a few years back in FHB where they didn't want a "ceiling" it was on the cover... seems like a sq room with a weird roof...
pony
I had something similar several years ago. I made up a center nailer ( that's what I call it ). I cut three octaganal pieces from a 2x12, then ripped a 2x8 on a 22 1/2* angle. I crosscut the 2x8 into 12" lengths, then nailed and glued to the three octagon forms. One at top and bottom ,and third one in middle. Rafter ties can then be mounted to the center nailer.
mike
i'm having a little trouble visualizing your technique. can you clarify?
Instead of the rafters meeting together at one intersecting point, an octagan hub was constructed out of 2x material. Each rafter frames into one side of the octagon hub and connected with steel angles on each side of each rafter. Sort of like taking a short piece of timber and beveling each side to form an octagon.The rafter has the plumb cut but not a cheekcut. If framed without the hub you would have four rafters with double cheek cuts , tough to fasten correctly. The hub allows the framer to connect all eight rafters with steel angles. The roof I worked on was an octagon with curved rafters, glad I didn't have to sheath it.
A Yurt is held up by the same principle as a tension ring.
It makes no differenc whether you surrond the plate system with a ring to share loading or use hardware or glue and gussets at all eight plate joints to keep the whole ring sungular. Both would work.
Personally, I prefer the idea of a tension ring. With hadware, you are depending on the fasteners and that the wood doesn't ever split.
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